A family of viruses, the bacteriophages, has the particularity of being both a harmful enemy which contributes to antibiotic resistance in bacteria, but also a valuable ally in the fight against bacterial infections. Understanding the complex biological functioning of these viruses is essential to be able to develop alternatives to antibiotics.
Bacteriophages (or simply phages) are viruses that have the ability to infect bacteria and, in most cases, kill them. They represent the most abundant biological entity on Earth and are present everywhere. There would be ten times more phages than bacteria on the planet. The latter are essential to the biosphere since they ensure its balance by avoiding any disproportionate and uncontrolled proliferation of bacteria.
Like all viruses, they need to infect a host cell to reproduce. When infecting a bacterium, they can adopt two different reproduction cycles: the lytic cycle and the lysogenic cycle. The lytic cycle allows phages to hijack the cellular machinery to produce new viral particles. The accumulation of the latter causes the wall of the infected bacteria to rupture, which releases new phages which are free to infect other cells in turn. Lytic phages are the most aggressive and lead to the rapid death of infected cells.
Phage therapy eclipsed by antibiotics
Thanks to their ability to kill bacteria rapidly and in a targeted manner, lytic phages have been used as a therapeutic strategy against bacterial infections: phage therapy. The latter showed its effectiveness years before the discovery of penicillin to treat cases of plague, dysentery and cholera at the beginning of the 20th century. But phage therapy was quickly eclipsed by the democratization of antibiotics, which are less expensive and easier to produce.
The lysogenic cycle is, for its part, often considered as a “dormancy” cycle where the phage introduces its viral genome (or prophage) into the bacterial genome without killing the host cell. When the prophage is activated by external stimuli (such as ultraviolet light), it extrudes from the host cell’s DNA, multiplies, and enters a lytic cycle as previously described. However, sometimes the prophage begins to multiply before extruding from the host DNA.
By multiplying, it replicates more or less long fragments of the host bacterium’s DNA in addition to replicating its own genome. These bacterial DNA fragments can end up encapsulated in the new phages and be transmitted to other bacteria during a new cycle of infection. This is the phenomenon of lateral transduction, the lysogenic cycle is therefore responsible for the horizontal transfer of genes. In other words, lysogenic phages are capable of transmitting genes between bacteria without these being direct descendants.
The massive and systematic use of antibiotics in recent decades has favored the emergence of resistance genes and lysogenic phages intensively contribute to the rapid spread of these antibiotic resistance genes through bacterial populations by transduction via prophages. There is therefore a duality between lytic phages, precious allies in the fight against bacterial infections, and lysogenic phages, harmful enemies which actively participate in the increase in the number of bacterial strains resistant to antibiotics.
How do phages recognize the cells to be infected?
Whether lytic or lysogenic, a phage only infects certain bacteria in a targeted and very specific way. Part of our work carried out at the Laboratory of Physical Chemistry and Microbiology for Materials and the Environment (LCPME) of the University of Lorraine-CNRS aims to understand how phages recognize their host bacteria in order to infect them.
Infection begins with the recognition of a suitable host cell by proteins on the surface of the phage which specifically bind to certain receptors on the surface of the bacteria and which allow the adhesion of the phage to the host.
No infection is possible without adhesion. The recognition of specific receptors by the phage is therefore a key step. However, many gray areas persist regarding its operation and few receptors have been clearly identified.
It is in this context that our scientific article published in the journal “Nano Research” in July 2022 comes into play.
A new technique
In this publication, we used atomic force microscopy (AFM) to identify phage 187 receptors (Figure 1a) that lead to infection of Staphylococcus aureus (Staphylococcus aureus, Figure 1b). S. aureus is one of the three main pathogens responsible for nosocomial infections which affect approximately one in twenty patients in France.
We propose a new method, Single-Particle Force Spectroscopy (SPFS) to characterize at the nanoscale the adhesion forces between phages and bacteria.
For this, we have grafted and immobilized a phage on the surface of a pyramidal point whose top measures only a few nanometers.
The tips were used to scan the surfaces of live bacteria while recording force curves. The latter make it possible to extract the adhesion forces that bind the phage to the surface of the bacteria. The more membership, the stronger the recognition.
Thanks to this technique, we have identified a sugar called N-acetylglucosamine which is a carbohydrate present in the cell walls of S. aureusas the main receptor involved in the recognition and therefore the infection of S. aureus by phage 187.
The use of single virus particle force spectroscopy opens new perspectives for the identification at the nanometric scale of receptors without which the infection of bacteria would not take place. This is a proof of concept of a technique to improve our understanding of the complex biological interactions of phages and which can help researchers to develop new effective treatments, to use the transduction properties of phages to our advantage and to limit the transfer of antibiotic resistance genes.
This analysis was written by Baptiste Arbez, post-doctoral researcher in materials for health in the laboratory of physical chemistry and microbiology, and Sofiane El-Kirat-Chatel, researcher in atomic force microscopy, bio-interfaces and microbiology (both at the University of Lorraine).
The original article was published on the site of The Conversation.